Headless dinosaurs

Sauropod dinosaurs—like the Diplodocus (dip-LOW-duh-cus) in the Dinosaurs and Fossils Gallery—had big bodies and teeny heads. Their fragile skulls were paper-thin in places. This means sauropod heads are among the rarest of dinosaur finds. Paleontologists have found about a dozen skulls from the Jurassic Period (208-144 million years ago), but no complete Cretaceous Period (144-65 million years ago) skulls in North America. Until now.

Scientists at Dinosaur National Monument in Utah are studying four Cretaceous sauropod skulls found in their quarry over the past few years. All four are the same type, from a new species that lived about 100 million years ago. And the skulls are unusually well preserved. (Sauropod skulls aren't single bones, but collections of very delicate bones. When the animals died, the heads usually separated from the bodies and the skulls fell apart or scavengers scattered them. The fragile pieces usually didn't fossilize.)

Sauropods were very common in the middle part of the age of dinosaurs, the Jurassic Period. But they became rarer in the Cretaceous. The skulls may offer valuable clues to the evolution and eventual extinction of this impressive group of beasts.

A lucky break
The paleontologists at Dinosaur National Monument found one extremely well preserved and articulated (not in pieces) skull, a second disarticulated skull (with all its pieces), the snout from a third animal, and the brain case of a fourth. Who knows? There may be many more in the quarry.

How did four skulls end up together? Scientists can't know for sure. The quarry site was a stream or river 100 million years ago. Maybe a herd of dinosaurs was crossing the river and these animals drowned. Or maybe they died during a drought, waiting beside a dried-up river, and a later flash flood washed all the bones together.

Poison Creek Diplodocus Skull

The Minnesota connection? Nature's little miracle...
Science Museum paleontologists found this intact juvenile Diplodocus skull at Poison Creek Quarry, near Buffalo, Wyoming. It's about 150 million years old, and from the Jurassic Period, not the Cretaceous. The thin, juvenile bones make this skull even more delicate than other sauropod skulls and an even luckier find. It's so well preserved that you can see a row of small replacement "bud" teeth coming in along the upper jaw.

The remains of other dinosaurs were also found in the quarry—including the adult Diplodocus, one of the Camptosaurs, and the Apatosaurus femur on display in the Dinosaurs and Fossils Gallery. 150 million years ago, the quarry was a flood plain. The bodies of the dinosaurs that died there piled up in quiet backwaters, and sediments buried the bones and preserved them. Because so many skeletons from the quarry were still articulated, we know that the water wasn't moving fast and scavengers didn't disturb the bodies. These conditions also helped to preserve this skull.

A treasure map
This is a section of SMM paleontologist Bruce Erickson's map of the Poison Creek quarry. You can just see the snout of the Diplodocus skull peeking out from under the articulated neck bones of an adult Diplodocus. (The adult skeleton shown in this map is the one mounted in the Dinosaurs and Fossils Gallery.)

Temporal fenestrae are holes in the head. The term comes from two Latin words, meaning "temples (of the head)" and "window."

All dinosaurs have two such holes on each side of the skull. Mammals have only one. You, as a mammal, have one yourself--the gap between your skull and your cheekbone, where chewing muscles are attached.

As for why dinosaurs have these holes, there are several theories:

When an animal chews, its muscles pull on the bone, creating stress. Bone grows more thickly where it is stressed, and is thin or absent in other areas. The distribution of stress in a dinosaur skull was such that no bone was needed in these areas.

Muscles can be more strongly anchored to edges of bone than to flat surfaces. The holes created places to attach powerful chewing muscles.

It takes energy to grow bone. Less bone means you use less energy.

Growing less bone means you use less calcium, which can then be put to other uses.

Holes make the skull lighter, and easier to carry.

(Much of this information comes from Dinosaurs: the Textbook, by Spencer G. Lucas; Wm. C. Brown Publishers, 1994. A quick Yahoo search on "temporal Fenestrae dinosaurs" yielded a couple of readable lectures, by Thomas Herbert of the University of Miami and Steven Wallace of East Tennessee University, which cover much of the same ground.)

So, which theory is correct? I dunno. All of them have some evidence to back them up. It's likely that several of these factors--perhaps even all of them--played some role in giving dinosaurs holes in the head.

First, we need to define some terms. Scientists use the word "dinosaur" to refer to a group of animals who all shared certain features, particularly the holes in the skull, the hinge in the ankle, and the structure of the hip. This indicates that they all descended from a single ancestor.

(Lay people often use the word "dinosaur" to mean, as my brother put it, "anything big, old and dead!" That covers a lot of ground. And while it may be useful shorthand, it doesn't match the evolutionary relationships we use to categorize animals.)

Crocodiles are close relatives of dinosaurs. They share many features, including the holes in the head. Dinos and crocs both evolved from a common ancestor about 260 million years ago. But the crocs evolved in one direction, and retained their primitive features in the ankle and hip; while dinosaurs evolved in a different direction until, about 235 million years ago, the first true dinosaur emerged. (All dates from Dr. David Norman's The Illustrated Encyclopedia of Dinosaurs.)

It's as if your grandfather had a sister. Your grandpa settled in Minnesota; his sister went to LA. 75 years later, you've got a bunch of cousins on the West Coast. They've probably got different names than you, and different family traditions, and so on. You can all trace your ancestry back to your GREAT-grandparents, to show that you are related. But you are definitely two different families.

So, no, crocodilians are not now, nor have they ever been, members of the Dinosaurian party.

None of these groups were closely related to crocodiles or dinosaurs. In fact, crocs and dinos were cousins -- though they evolved in different directions, they shared a common ancestor about 260 million years ago.

Mososaurs were part of the lizard family. The lizard group split from the croc/dino branch about 265 million years ago.

Plesioaurs and ichthyosaurs are even more distant. They last shared an ancestor with crocs, dinos and lizards almost 300 million years ago.

So, crocodiles have their own branch of the reptile family tree. Their only close relatives were the phytosaurs, who died out shortly after the dinosaurs emerged.

On the other hand, crocodiles have been around from 260 million years, and have out-lived a lot of other groups. They must be doing SOMETHING right!

The group of reptiles that are ancestral to both dinosaurs and crocodilians are called thecodonts. Thecodonts were also ancestral to pterosaurs (flying reptiles). Crocodilian fossils date back to the Triassic Period, over 200 million years ago, just as the dinosaurs were appearing. The earliest crocodilians would not have looked very familiar to us. They resembled their thecodont ancestors. Not all of them were amphibious, and some probably moved quickly over land with their long, slender legs. Early dinosaurs looked much like their thecodont ancestors, too. So much, in fact, that historically, paleontologists haven't always agree on where to draw the line (advanced thecodont or early dinosaur?). There are many fossil thecodonts known, several of which are probably close to the true ancestors of both crocodiles and dinosaurs. But it is unlikely that we'd ever be able to say which exact species of thecodont gave rise to which exact species of early dinosaur or crocodile.

First, I bewlieve you need more than DNA to clone an animal. You need a living cell. And it's not like cloning is a simple, backyard project -- it's very complicated, and even with modern animals fails more often than succeeds.

That's always a great feeling to find a complete dinosaur skeleton. Regarding the DNS, well, it is still a bit of sci-fi yet theoretically it is possible to replicate a dinosaur using a living elephant or crocodile. And it is possible even with an incomplete DNS because a computer program can replicate missing parts of a DNS. Well, I hope you won't try to do this - we don't want a new breed of hybrid monsters roaming the forests :)

Check out this very illuminating article. It addresses the question of just how dinosaurs must have done... it. It's tricky business; we're dealing with creatures weighing perhaps a hundred tons in some cases, and on top of the occasional spikes and armor, they had inconveniently-located tails as thick as tree trunks. For dinosaurs to have mated the same way mammals generally do, they'd have needed, as one scientist puts it, "penises 3 meters long and shaped like corkscrews." Unfortunately, that's not the sort of thing that fossilizes well. And it doesn't seem likely in the first place.

What's more probable is that dinosaurs had cloacae similar to today's reptiles and birds, and mating would have required some delicate, sexy Mesozoic Twister to match them up. (Cloacae, by the way, are sort of all-purpose in-and-out ports. Well, not all purpose. They aren't for eating and breathing, if you know what I mean.)

The article also suggests that some of the larger dinosaurs (sauropods weighing dozens of tons) may have made use of bodies of water to provide a little buoyancy. (Removing some of that weight could have lessened the risk of crushing, and just made things easier in general.)

Um. The answer you might have been looking for is, "Diplodocus laid eggs. Unlike some other dinosaurs, which laid their eggs in nests, diplodocus appears to have laid its eggs in a long line, perhaps while walking. It's not thought that diplodocus hung around to take care of its eggs and newly hatched babies... but we don't know for sure."

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